The present invention is related to a nonwoven fabric containing conjugate microfilaments. More particularly, the present invention is related to a nonwoven fabric containing pneumatically drawn conjugate microfilaments.
Synthetic filaments having an average thickness, more specifically weight-per-unit-length, of about 1.5 dtex or less can be characterized as microfilaments, and two commonly used groups of processes for producing microfilaments are meltblown fiber production processes and split fiber production processes. Meltblown fibers are formed by extruding a melt-processed thermoplastic material through a plurality of fine die capillaries as molten filaments into a high velocity heated gas stream, typically heated air, which attenuates the filaments of molten thermoplastic material to reduce their diameter to form meltblown fibers. The fibers, which typically are tacky and not fully quenched, are then carried by the high velocity gas stream and randomly deposited on a collecting surface to form an autogenously bonded web. Meltblown webs are widely used in various applications such as filters, wiping cloths, packaging materials, disposable clothing components, absorbent article components and the like. However, the attenuating step of the meltblown fiber production process imparts only a limited level of molecular orientation in the polymer of the forming fibers, and thus, meltblown fibers and webs containing the fibers do not exhibit high strength properties.
Split fibers, in general, are produced from a multicomponent conjugate fiber which contains typically incompatible polymer components that are arranged to occupy distinct zones across the cross-section of the conjugate fiber and the zones are extended along the length of the fiber. Split fibers are formed when the conjugate fiber is mechanically or chemically induced to split along the interface of the distinct zones within the fiber. Although a split fiber production process can be used to produce fine fibers having relatively high strength properties, the process requires the splitting step and the step tends to be cumbersome and costly. In addition, it is highly difficult to produce completely split fibers from conventional split fiber production processes, and these processes tend to produce compacted or densified structures.
There have been attempts to produce microfilaments that are subsequently cut to form staple fibers. Such microfilaments are produced by forming filaments through spinning apertures of a spinneret and then drawing the filaments, typically with take-up rolls, at a high drawing speed to apply a high drawing ratio. However, as the thickness of microfilaments gets finer, microfilaments and micro staple fibers produced therefrom create processing difficulties. For example, micro staple fibers are highly difficult to open and card, and the fibers tend to form non-uniform nonwoven webs when carded.
Alternatively, there have been attempts to produce microfilament nonwoven webs by modifying spunbond nonwoven web production processes. Spunbond filaments are formed, analogous to a meltblown fiber production process, by melt-processing a thermoplastic polymer through a plurality of fine die capillaries to form molten filaments. Unlike a meltblown fiber production process, however, the formed filaments are not injected into a heated gas stream but are conveyed to a pneumatic drawing unit while being cooled, and drawing forces are applied on the filaments with pressurized gas or air in the pneumatic drawing unit. The drawn filaments exiting the drawing unit, which are relatively crimp-free filaments, are deposited onto a forming surface in random manner to form a loosely entangled fiber web, and then the laid web is bonded under heat and pressure to form melt fused bonded regions in order to impart web integrity and dimensional stability. Spunbond filaments have relatively high molecular orientation, compared to meltblown fibers, and thus exhibit relatively high strength properties. However, spunbond nonwoven webs tend to be compacted and flat due to the uncrimped nature of the spunbond filaments and the compaction bonding process. The production of spunbond webs is disclosed, for example, in U.S. Pat. Nos. 4,340,563 to Appel et al.; 3,692,618 to Dorschner et al. and 3,802,817 to Matsuki et al.
In order to improve the bulk of spunbond webs, production of crimped filament spunbond webs has been proposed. For example, U.S. Pat. No. 5,382,400 to Pike et al. teaches a spunbond web production process which produces lofty spunbond webs containing multicomponent conjugate filaments. The teaching of U.S. Pat. No. 5,382,400 is highly suitable for producing lofty spunbond webs. However, attempts to produce lofty webs containing finer filaments than conventional spunbond filaments have not been highly successful. It has been found that increasing the pneumatic drawing force and/or reducing the throughput rate of the melt-processed polymer into the die capillaries, which are conventional production means for reducing the thickness of the filaments, substantially eliminate crimps in the fine conjugate filaments. In addition, it has been found that the application of the known means to reduce the size of spunbond filaments does not indefinitely reduce the size of the filaments. As the pneumatic drawing force is increased and/or the throughput rate is decreased to a certain limit, severe spin breaks disrupt the spinning process altogether. Consequently, there is a significant limit in reducing the thickness of spunbond filaments using the conventionally known means, and producing crimped spunbond microfilaments with a conventional spunbond filament production approach is not practicable.
There remains a need for a microfilament nonwoven web that is lofty and has high strength properties.